Windows Phone 7 异步编程模型
Windows Phone 7上的异步编程模型其实也就是说把C#里面的异步编程模型在Windows Phone 7应用开发上使用。下面来看一下异步编程模型里面的一些关键的概念。
2个方法和一个委托和一个接口：
(1)BeginInvoke方法用于启动异步调用
Begin 方法包含同步方法签名中的任何参数，此外还包含另外两个参数：一个AsyncCallback 委托和一个用户定义的状态对象。委托用来调用回调方法，状态对象是用来向回调方法传递状态信息。
BeginInvoke立即返回，不等待异步调用完成。
BeginInvoke返回IasyncResult，可用于监视调用进度。
(2)EndInvoke方法用于检索异步调用结果。
End 方法用于结束异步操作并返回结果，因此包含同步方法签名中的 ref 和 out 参数，返回值类型也与同步方法相同。该方法还包括一个 IAsyncResult 参数，用于获取异步操作是否完成的信息，当然在使用时就必须传入对应的 Begin 方法返回的对象实例开始异步操作后如果要阻止应用程序，可以直接调用 End 方法，这会阻止应用程序直到异步操作完成后再继续执行。也可以使用 IAsyncResult 的 AsyncWaitHandle 属性，调用其中的WaitOne等方法来阻塞线程。这两种方法的区别不大，只是前者必须一直等待而后者可以设置等待超时。
如果不阻止应用程序，则可以通过轮循 IAsyncResult 的 IsCompleted 状态来判断操作是否完成，或使用 AsyncCallback 委托来结束异步操作。AsyncCallback 委托包含一个 IAsyncResult 的签名，回调方法内部再调用 End 方法来获取操作执行结果。
在调用BeginInvoke后可随时调用EndInvoke方法,注意：始终在异步调用完成后调用EndInvoke.如果异步调用未完成，EndInvoke将一直阻塞到异步调用完成。
EndInvoke的参数包括需要异步执行的方法的out和ref参数以及由BeginInvoke返回的IAsyncResult。要注意的是,始终在异步调用完成后调用EndInvoke
(3)AsyncCallback委托用于指定在开始操作完成后应被调用的方法
AsyncCallback委托被作为开始操作上的第二个到最后一个参数传递
代码原型如下：
[Serializable]
public delegate void AsyncCallback(IAsyncResult ar);
(4)IAsyncResult接口
它表示异步操作的状态.
该接口定义了4个公用属性
public interface IAsyncResult
{
object AsyncState { get; }
WaitHandle AsyncWaitHandle { get; }
bool CompletedSynchronously { get; }
bool IsCompleted { get; }
}
在Windows Phone 7上的应用
AsyncResultNoResult.cs
using System; using System.Threading; namespace AsyncTaskDemo { public class AsyncResultNoResult : IAsyncResult { private readonly AsyncCallback _asyncCallback;//异步请求的返回方法 private readonly object _asyncState;//异步请求的对象 private ManualResetEvent _asyncWaitHandle;//线程阻塞 private Exception _exception;//操作异常 private int _completedState;//完成状态 private const int StateCompletedAsynchronously = 2;//异步完成 private const int StateCompletedSynchronously = 1;//同步完成 private const int StatePending = 0; /// <summary> /// 初始化 /// </summary> /// <param name="asyncCallback">异步返回方法</param> /// <param name="state">异步调用对象</param> public AsyncResultNoResult(AsyncCallback asyncCallback, object state) { this._asyncCallback = asyncCallback; this._asyncState = state; } /// <summary> /// 结束任务 /// </summary> public void EndInvoke() { // This method assumes that only 1 thread calls EndInvoke // for this object if (!this.IsCompleted)//任务未完成 { // If the operation isn't done, wait for it this.AsyncWaitHandle.WaitOne(); this.AsyncWaitHandle.Close(); this._asyncWaitHandle = null; // Allow early GC } if (this._exception != null) { RethrowException(this._exception); } } private static void RethrowException(Exception ex) { throw ex; } public void SetAsCompleted(Exception exception, bool completedSynchronously) { // Passing null for exception means no error occurred. // This is the common case this._exception = exception; // The m_CompletedState field MUST be set prior calling the callback if (Interlocked.Exchange(ref this._completedState, completedSynchronously ? 1 : 2) != 0) { throw new InvalidOperationException("You can set a result only once", exception); } if (this._asyncWaitHandle != null) {// If the event exists, set it this._asyncWaitHandle.Set(); } if (this._asyncCallback != null) {// If a callback method was set, call it this._asyncCallback(this);//调用异步请求的返回方法 } } /// <summary> /// 放弃异步请求的对象 /// </summary> public object AsyncState { get { return this._asyncState; } } public WaitHandle AsyncWaitHandle { get { if (this._asyncWaitHandle == null) { bool isCompleted = this.IsCompleted; ManualResetEvent event2 = new ManualResetEvent(isCompleted); if (Interlocked.CompareExchange<ManualResetEvent>(ref this._asyncWaitHandle, event2, null) != null) { // Another thread created this object's event; dispose // the event we just created event2.Close(); } else if (!isCompleted && this.IsCompleted) { // If the operation wasn't done when we created // the event but now it is done, set the event //如果操作已经完成则释放阻塞 this._asyncWaitHandle.Set(); } } return this._asyncWaitHandle; } } //同步操作是否已经完成 public bool CompletedSynchronously { get { return (this._completedState == 1); } } //是否已经完成 public bool IsCompleted { get { return (this._completedState != 0); } } } }
AsyncResult.cs
using System; namespace AsyncTaskDemo { public class AsyncResult<TResult> : AsyncResultNoResult { private TResult m_result;//异步操作完成返回的对象 public AsyncResult(AsyncCallback asyncCallback, object state) : base(asyncCallback, state) { this.m_result = default(TResult); } /// <summary> /// 停止异步返回返回对象 /// </summary> /// <returns></returns> public TResult EndInvoke() { base.EndInvoke(); //等待操作完成 return m_result; // 返回结果 } /// <summary> /// 操作完成 /// </summary> /// <param name="result">返回结果对象</param> /// <param name="completedSynchronously">是否同步操作</param> public void SetAsCompleted(TResult result, bool completedSynchronously) { //保存异步操作的结果 m_result = result; base.SetAsCompleted(null, completedSynchronously); } } }
TestTask.cs
using System; using System.Threading; namespace AsyncTaskDemo { public class TestTask { public TestTask() { } public IAsyncResult BeginTestTask(string text, AsyncCallback asyncCallback, object state) { AsyncResult<string> asyncResult = new AsyncResult<string>(asyncCallback, state); ThreadPool.QueueUserWorkItem(DoTestTask, asyncResult); return asyncResult; } public string EndTestTask(IAsyncResult asyncResult) { return ((AsyncResult<string>)asyncResult).EndInvoke(); } private void DoTestTask(Object asyncResult) { AsyncResult<string> ar = (AsyncResult<string>)asyncResult; try { Thread.Sleep(3000); ar.SetAsCompleted("测试完成", true); } catch (Exception e) { ar.SetAsCompleted(e, false); } } } }
MainPage.xaml.cs
TestTask tt = new TestTask(); WebTask webTask = new WebTask(new Uri("http://www.cnblogs.com/linzheng")); IAsyncResult ia; public MainPage() { InitializeComponent(); } private void button1_Click(object sender, RoutedEventArgs e) { Func<string, IObservable<string>> testResult = Observable.FromAsyncPattern<string, string>(tt.BeginTestTask, tt.EndTestTask); ObservableExtensions.Subscribe<string>( Observable.ObserveOnDispatcher<string> ( testResult.Invoke("") ), delegate(string success) { MessageBox.Show(success); }, delegate(Exception exception) { MessageBox.Show(exception.Message); } ); }
运行的效果
